The present invention relates to a bottom bracket assembly for a bicycle and, more particularly, to an adjustable bottom bracket assembly for adjusting the tension of a flexible drive member such as a chain or a belt of the bicycle drive train.
The primary structural component of a conventional two-wheel bicycle is the frame. On a conventional road bicycle, the frame is typically constructed from a set of tubular members assembled together to form the frame. For many bicycles, the frame is constructed from members commonly referred to as the top tube, down tube, seat tube, seat stays and chain stays, and those members are joined together at intersections commonly referred to as the head tube, seat post, bottom bracket tube or shell and rear dropout. The top tube usually extends from the head tube rearward to the seat tube. The head tube, sometimes referred to as the neck, is a short tubular structural member at the upper forward portion of the bicycle which supports the handlebar and front steering fork, which has the front wheel on it. The down tube usually extends downwardly and rearward from the head tube to the bottom bracket shell. A bottom bracket assembly usually comprises a cylindrical tubular member or spindle for supporting the pedals and corresponding crankset or chainset which cooperate with the flexible drive mechanism or chain to power the bicycle. The bottom bracket assembly commonly slidably cooperates with the bottom bracket shell of the bicycle frame assembly. The seat tube usually extends from the bottom bracket assembly upwardly to where it is joined to the rear end of the top tube. The seat tube also usually functions to telescopically receive a seat post for supporting a seat or saddle for the bicycle rider to sit on.
The chain stays normally extend rearward from the bottom bracket shell. The seat stays normally extend downwardly and rearward from an upper end of the seat tube. The chain stays and seat stays are normally joined together at a rear dropout and cooperate to support the opposite ends of a rear axle associated with the rear wheel. The portion of the frame defined by the head tube, seat post and bottom bracket shell and the structural members that join those three items together is commonly referred to as the main front triangular portion of the frame, with the seat stays and chain stays defining a rear triangular portion of the frame. Understandably, the above description is merely exemplary of one bicycle frame assembly useable with the present invention.
The bottom bracket assembly commonly includes a spindle that is associated with the crankset of the bicycle and allows the crankset to rotate freely relative to the bottom bracket shell when the flexible drive member is dissociated from the drive train. A bearing is commonly associated with one or both of the opposite ends of the spindle and rotationally supports the spindle and corresponding crankset relative to the bottom bracket shell. One of more gears or chain rings and the crankarms associated with the pedals are attached to the spindle such the rotation of the pedals rotates the chain rings which in turn drive the flexible drive members. Commonly, the spindle rotationally cooperates directly with the cavity defined by bottom bracket shell or tube of the bicycle frame. Proper operation of the bicycle drive train relies in part on the proper tensioning of the flexible drive member associated with a respective drive and driven gear or chain ring or crankset gear associated with the drive train.
Commonly, the flexible drive member such as a chain or belt extends between one or more drive and driven members or chain rings associated with the crankset and a gear cassette associated with the driven wheel. Inadequate tensioning of the flexible drive member can result in the flexible drive member becoming disengaged or derailed from desired engagement with a respective drive or driven ring. Commonly, the tension of the flexible drive member is only periodically adjusted via manipulation of the rear wheel relative to an elongated slot that defines the dropout of the driven wheel. During use of a bicycle, the chain or belt can become worn or “stretched” which can result in undesired derailing of the flexible drive member if the bicycle is not properly maintained. Accordingly, it is occasionally desired to monitor and/or adjust the tension of the flexible drive member to ensure desired operation of the drive train.
Rather than manipulating the position of the rear wheel assembly relative to the bicycle frame, other provide means of adjusting the tension of the flexible drive member via manipulation of the bottom bracket assembly relative to the bottom bracket shell of the frame. One type of such a bottom bracket assembly is an eccentric bottom bracket assembly that allows selective translation of the axis of rotation of the spindle relative to the bottom bracket shell. In an eccentric bottom bracket assembly, a cylindrical plug includes a cavity that accommodates the spindle and is shaped to fit into the cylindrical cavity defined by the bottom bracket shell. The plug is commonly machined to accept a spindle such that the axis of rotation of the spindle is offset from the longitudinal centerline of the plug so that rotation of the plug translates the axis of rotation of the spindle relative to the shell so that the axis of rotation of the crankset can be selectively moved fore or aft. Since the chain drive (or other flexible drive) is interconnected between the crankset and a wheel assembly (typically, the rear wheel assembly), this fore or aft movement of the bottom bracket adjusts the tension associated with operation of the flexible drive member or chain. Unfortunately, such systems are not without their drawbacks.
Commonly, such systems include a pair of wedging portion that cooperate with one another to selectively fix the position of the bottom bracket assembly relative to the bottom bracket shell via manipulation of one or more fasteners that translate the wedge portions into one another and thereby compress against the interior contour of the bottom bracket shell. When loose, such assemblies provide no control as to the orientation of the bottom bracket assembly relative to the bottom bracket shell aside from user interaction with crankarms or pedals of the crank assembly. Thus, although such eccentric bottom bracket assemblies allow manipulation of the tension of the flexible drive member, such assemblies do so in a manner that is less than precise and is susceptible to less than desired tensioning of the flexible drive member as a function of the hand, arm, and holding strength of the user or technician performing the adjustment. Accordingly, there is a need for a bicycle flexible drive member adjustment mechanism that can precise adjust the tension of the flexible drive member and does so in a manner that is less susceptible to individualization associated with the skill and strength of the person performing the adjustment.